1. Field of the Invention
The Present Invention relates generally to capacitive type sensor apparatus, and more particularly to a method and a capacitive type "Capaciflector" proximity sensor for sensing the type of material in the object being approached by the sensor.
2. Description of the Prior Art
Capaciflector type proximity sensors are well known having been first disclosed in U.S. Pat. No. 5,166,679, entitled "Driven Shielding Capacitive Proximity Sensor", issued to John M. Vranish on Nov. 24, 1992. There, a capacitive type proximity sensing element is backed by a reflective shield, with the shield being driven by an operational amplifier (op-amp) at the same voltage as and in phase with the sensing element. The shield is used to reflect the sensing element's electric field lines away from a grounded surface towards an intruding object and thus substantially increasing the sensor's range and sensitivity. The output signal from the sensing element is coupled to an oscillator circuit whose output is inversely proportional to the capacitance between the sensor and ground. Such a sensor system is relatively simple and yet has the ability to sense virtually any object it encounters.
In the related U.S. patent application U.S. Ser. No. 08/189,344 entitled "Current-Measuring OP-AMP Devices," referenced above, there is disclosed an electrical circuit called a current-measuring voltage follower which is in effect a micro miniature precision high-gain servo system in which voltage applied to one terminal of an operational amplifier (op-amp) is servoed through the op-amp to its other input terminal. The op-amp acts as a current source having a low impedance output. Therefore when such a current-measuring voltage follower circuit is inserted between a voltage source and the sensor element and the shield of a capaciflector proximity sensor, the current through the sensor will vary according to what it senses and is measured thereby.
Prior art capaciflector circuits also operate at single frequencies, i.e. the capaciflector operates at a set frequency, although the selected frequency can differ between different capaciflectors. Thus it provides capacitance measurements at a single frequency and although it is affected by the permittivity of nearby objects it does not provide sufficient information to identify the target object's material composition. Thus know prior art capaciflector systems cannot identify materials by their composition.
Known prior art apparatus for detecting the composition of material typically includes such apparatus as a gamma ray spectrometer. This type of device utilizes radiation to obtain a characteristic excitation curve which is used to identify materials by their elemental composition. A gamma ray spectrometer, however, takes a relatively long period of time, e.g. 10 minutes, of "staring" at a target to acquire its data. Furthermore, different materials may have the same elemental composition such as sand and quartz and thus appears to a gamma ray spectrometer as the same thing. Another problem with a gamma ray spectrometer is its radioactive nature.
Also, the concept of frequency scanning is generally well known and is utilized in such apparatus as radar where it is referred to as "chirping". Radar, however, is used for distance detection using far field techniques and does not operate in the near field.